Embodiments of the invention relate to a microelectromechanical system (MEMS) pressure sensing element having a stepped cavity at the backside for reducing or eliminating thermal noise induced by thermal stresses, such as the temperature coefficient of offset voltage output (TCO).
MEMS pressure sensors are generally known. One type of pressure sensor is a differential pressure sensor, which includes a silicon pressure sensing element that is anodically bonded to a glass pedestal and mounted to a housing substrate using an adhesive. Many differential pressure sensors are used in applications in which the sensors are exposed to varying temperatures. This causes the sensing element, the glass pedestal, the adhesive, and the housing substrate to expand and contract in response to the temperature changes.
The pressure sensing element includes four piezoresistors or resistors positioned in what is known as a “Wheatstone Bridge” configuration to sense the stresses that are applied to the resistors. The glass pedestal is incorporated between the pressure sensing element and the adhesive such that the stresses resulting from the difference in thermal expansion among the pressure sensing element, the adhesive, and the housing substrate are isolated by the glass pedestal. The glass pedestal and the pressure sensing element have slightly different coefficients of thermal expansion, and therefore expand and contract at a lower different rate when exposed to varying temperatures. The glass pedestal essentially acts as a buffer to isolate the stresses resulting from the different expansion and contraction rates among the pressure sensing element, the adhesive, and the housing substrate.
An example of the pressure sensor discussed above is shown in FIG. 1 generally at 10. The sensor 10 includes a pressure sensing element 12, a glass pedestal 14, an adhesive 16, and a housing substrate 18. The pressure sensing element 12 shown in FIG. 1 is made from silicon, and is anodically bonded to the glass pedestal 14. The adhesive 16 is used to bond the glass pedestal 14 to the housing substrate 18.
Formed as part of the housing substrate 18 is a first aperture 20, and formed as part of the glass pedestal 14 is a second aperture 22, which is in substantial alignment with the first aperture 20. The second aperture 22 is in fluid communication with a cavity, shown generally at 24, where the cavity 24 is formed as part of the pressure sensing element 12. The pressure sensing element 12 includes four angular inner surfaces, where only a first angular inner surface 26 and a second angular inner surface 28 are depicted in the cross-sectional view of FIG. 1. Each of the four angular inner surfaces terminates into a bottom surface 30, which is part of a diaphragm 32. The pressure sensing element 12 also includes a top surface 34, and there is a picture-frame transducer or picture-frame Wheatstone bridge 36 doped onto the top surface 34 of the pressure sensing element 12. At least a thermal oxide layer and passivation layers are formed to protect the circuitry. The picture-frame Wheatstone bridge 36 is formed by four p− piezoresistors 36A-36D as shown in FIG. 2B. The four piezoresistors 36A-36D may also be formed as a distributed Wheatstone bridge 38A-38D as shown in FIG. 3 for pressure sensing.
The diaphragm 32 is relatively thin in the micron range, and the thickness of the diaphragm 32 depends upon the pressure range. The diaphragm 32 deflects upwardly and downwardly in response to pressure applied to the bottom surface 30, and the top surface 34 of the diaphragm 32. The pressure in the cavity 24 changes as a result of a pressure change of fluid flowing into and out of the apertures 20 and 22.
The deflections on the top surface 34 also deform the picture-frame Wheatstone bridge 36, which is doped onto the top surface 34 of the pressure sensing element 12. The pressure sensing element 12 is made of a single-crystal silicon (Si). On the top of the pressure sensing element 12, four p− piezoresistors 36A-36D are formed and connected to each other by p+ interconnectors 40 to form the picture-frame Wheatstone bridge 36 for pressure sensing as shown in FIGS. 2A-2B.
As used herein, the term Wheatstone bridge refers to the circuit topology shown in FIG. 2A-2B, namely the parallel connection of two series-connected resistors.
FIGS. 2A-2B represent a top view of the piezoresistive pressure sensing element 12 with the picture-frame Wheatstone bridge 36, which is doped on the diaphragm 32. The diaphragm 32 has dimensions of 780 μm×780 μm. The thickness of the diaphragm 32 is generally in the range of about 5 μm to 20 μm. The picture-frame Wheatstone bridge 36 is processed using conventional techniques to form four resistors 36A-36D on the top surface of the pressure sensing element 12. The resistors 36A-36D are formed as p− resistors, embodiments of which are well-known to those of ordinary skill in the semiconductor art. Electrical interconnects 40 made of p+ material connected to the bottom of bond pads 42A-42D are also formed on the top surface 34 of the pressure sensing element 12. Each interconnect 40 provides an electrical connection between two resistors in order to connect the resistors to each other to form a piezoresistive Wheatstone bridge circuit.
The four interconnects 40 are shown as part of the pressure sensing element 12. Each interconnect 40 extends outwardly from a point or node 44 between two of the four resistors 36 next to each other, and connects to the bottom of a metal bond pad 42. Each bond pad 42 is located near a side 46 of the top surface 34 of the pressure sensing element 12. Each interconnect 40 thus terminates at and connects to a bond pad 42.
FIG. 2A also shows an orientation fiducial 48 on the top surface 34. The fiducial 48 is a visually perceptible symbol or icon the function of which is simply to enable the orientation of the pressure sensing element 12.
Each bond pad 42 has a different label or name that indicates its purpose. The first bond pad 42A and the second bond pad 42B receive an input or supply voltage for the Wheatstone bridge circuit. Those two bond pads 42A, 42B are denominated as Vp and Vn, respectively. The other two bond pads 42C, 42D are output signal nodes denominated as Sp and Sn, respectively.
Many attempts have been made to simplify the construction of this type of pressure sensor 10 by eliminating the glass pedestal 14, and directly mounting the pressure sensing element 12 to the housing substrate 18 with the adhesive 16. However, the difference in thermal expansion among the housing substrate 18, the adhesive 16, and the pressure sensing element 12 has resulted in unwanted stresses being applied to the pressure sensing element 12, which then disrupts each of the resistors 36A-36D, causing an inaccurate pressure reading by the pressure sensing element 12.
More particularly, both experimental measurement and computer simulations of the structure depicted in FIG. 1 show that connecting the pressure sensing element 12 directly to the housing substrate 18 creates offset voltage output and its variation over an operating temperature range due to asymmetrical thermal stresses on the resistors 36A-36D. Elimination of the glass pedestal 14 causes one of the resistors 36A through 36D to deform and to change its resistance value asymmetrically with respect to the other resistors leading to an offset voltage output variation in an operating temperature range in the output of the pressure sensing element 12.
The offset voltage output variation over an operating temperature is called temperature coefficient of offset voltage output (TCO) and defined as follows:TCO=(Vo at 150° C.−Vo at −40° C.)/190° C.
Where Vo at 150° C.: offset voltage output at 150° C. without pressure applied; and Vo at −40° C.: offset voltage output at −40° C. without pressure applied.
The pressure sensing element 12 is commonly used with an application-specific integrated circuit (ASIC). The ASIC is used for amplifying and calibrating the signal received from the pressure sensing element 12. It is desirable to keep the TCO between −50 uV/° C. and 50 uV/° C. so the ASIC is better able to handle any thermal noise.
It is difficult for an ASIC to compensate for a high TCO, especially when the adhesive 16 is not symmetrically dispensed. If the adhesive is not symmetrically dispensed, this can further reduce the accuracy of the sensor because the stress difference in the X and Y directions on each of the four resistors will be amplified. The difference between the offset voltage outputs at the low and high temperatures will, therefore, increase and so will the TCO. That is why the glass pedestal 14 shown in FIG. 1 is used to isolate the thermal stresses. In order to reduce cost and simplify the manufacturing process, it would be desirable to eliminate the glass pedestal. A pressure sensing element without a glass pedestal would also improve wire bonding stability and reliability. Therefore, a pressure sensor that does not have a glass pedestal and that has low TCO noise would advance the state of the art.